Dissociative chemisorption of water onto silica surfaces and formation of hydronium ions

Molecular dynamics (MD) computer simulation of the adsorption of water molecules onto the vitreous silica surface was performed using a new dissociative water potential.(58) The simulations showed dissociative chemisorption of water molecules onto the silica surface, forming silanol (SiOH) groups at a concentration consistent with experimental data. Water penetration and silanol formation similar to 7-8 angstrom below the outermost oxygen are observed. Because of the dissociative nature of the water potential, formation of hydronium ions is allowed, and, whereas seldom observed in the simulations of bulk water, hydronium ions are formed during the reactions causing the formation of the silanols. The formation of hydronium ions has also been observed in ab initio calculations of water adsorption onto silica surfaces. The time evolution of the reactions involving hydronium ions in our MD simulations is similar to that observed in first-principles MD calculations. Hydronium ions offer a mechanism by which initially singly coordinated terminal oxygen (Si-O-) receives a H+ ion from a relatively distant chemisorbed H2O molecule via multiple H+ ion transfer; creating two SiOH sites.